The present invention relates to a warning device. More particularly, it relates to a warning device comprising one or more sensing signal transmitting units and a single receiving unit electrically connected therewith wherein a sensor connected with the transmitting unit generates a sensing signal when, for example, a door under observation is opened, and the signal is transmitted to the receiving unit to provide a warning.
In particular, the warning device of the present invention comprises at least one sensing signal transmitting unit and a single receiving unit.
The transmitting unit includes a sensor which provides a sensing signal of a predetermined low frequency when, for example, a door under observation is opened. The transmitting unit further includes a low frequency oscillation portion which provides a low frequency signal in response to the sensing signal. The portion may be, for example, a resonance element. The low frequency signal is then frequency-multiplied and modulated to a high frequency signal by a modulation portion. A high frequency oscillation portion transmits the high frequency signal through a transmitting antenna.
The receiving unit in turn includes a receiving antenna to receive the high frequency signal from the transmitting unit, a frequency modulation portion which amplifies selectively the high frequency signal to an intermediate frequency signal, an intermediate frequency amplifying circuit which amplifies the signal, a detection circuit which detects a signal having the same frequency as that of the resonance frequency of the low frequency oscillation portion in the transmitting unit, a low frequency amplifying portion which receives the signal of the resonance frequency through a coupling condensor, a selection circuit which selects the output of the low frequency amplifying portion by, for example, the same resonance element as that in the transmitting unit, a low frequency amplifying circuit which amplifies the output signal of the selection circuit, a voltage doubler rectifying circuit which rectifies the output signal of the low frequency amplifying circuit to a direct current, and a driving circuit for warning which operates in response to a d.c. voltage resulting from the direct current.